Governor interlock valve

ABSTRACT

A transmission control wherein modulating solenoid valves provide for continued governor and accumulator trim boost in the event of a malfunction in either the electrical or the hydraulic system. The transmission control provides redundancy to permit continued operation in the event one of the modulating solenoid valves fails. A single interlock valve is employed to prevent the two modulating solenoid valves from pressurizing the governor and the trim boost circuits when the transmission is operating in the reverse drive range.

TECHNICAL FIELD

The present invention relates generally to transmission controls. Moreparticularly, the present invention relates to transmission controlsthat combine electrical and hydraulic operation. Specifically, thepresent invention relates to transmission controls which utilize asingle interlock valve to prevent the pulse width modulating valves frompressurizing the governor and trim boost circuits when the transmissionis operating in reverse.

BACKGROUND OF THE INVENTION

Power-shifting automatic transmissions of both the planetary type andthe countershaft type use hydraulically actuated torque transfer devicesto effect the selection of sequential drive ranges by selectivelyengageable friction members. Planetary type transmissions use frictiontorque transfer devices of both the clutch and brake variety.Countershaft type transmissions use friction torque transfer devices ofonly the clutch variety. The control mechanism which determines theshift sequence and timing for these transmissions can be eitherhydraulic control valving or the more recently introducedelectro-hydraulic control valving. With electro-hydraulic controls, apre-programmed digital computer is generally provided to determine boththe shift schedules and pressure levels of the hydraulic actuating fluidwithin the transmission. The computer employs a look-up table which hasthe necessary data to determine the shift points in response to inputsignals from vehicle parameter detectors, such as the vehicle and enginespeed sensors, engine torque level sensors, throttle position sensorsand the like.

The computer analyzes the input signals and refers to the look-up tableto determine the appropriate ratio interchange. The computer can alsoprovide the necessary control signals to establish the desired outputpressure of the solenoid valves. Generally, the solenoid valves areeither of the on-off type or the pulse width modulated (PWM) type. Witheither type, the output signal is delivered to either a valve, whichwill control the ratio interchange, or to the friction devices directly.

Currently employed control devices typically utilize a governor andthrottle signal to control the ratio interchange. In some instances,this signal is combined by the electronics to provide a singleelectrical output signal which will determine the output pressure of thesolenoid control valving. Should the solenoid valve have a malfunction,the transmission control includes a limp-home feature which causes thetransmission to select a fixed gear ratio until proper repairs areundertaken. This feature prevents the driver from being stranded due toan electrical or mechanical malfunction of the solenoids.

The purpose of the governor valve is to direct the higher of the twosolenoid pressures to the governor pressure passage and to the boostside of a plug valve incorporated in the trim boost valve. The governorvalve also directs the lower of the two solenoid pressures to the otherside of the plug valve in the trim boost valve. Trim boost pressure ismaintained at a level determined by a spring that is set by thedifferential pressure between solenoids which act on the plug through apin and stop structure. During upshifts, one solenoid is operated at alower level than the other solenoid so that a differential pressureexists on the trim boost plug resulting in the desired trim boostpressure. Between upshifts, the pressure of the solenoid operated at thelower level rises to the same level as the other solenoid. Interlockvalves provide the control that will "lock-out" a solenoid if amalfunction occurs which provides a pressure continuously greater thanzero. The "lock-out" is introduced when the transmission is shifted toreverse, thereby ensuring that the vehicle speed is essentially zero toprevent any unscheduled downshifts. The control will permit the operatorto resume normal operation although the upshifts will be harsh becauseof the high trim boost. This will remind the operator that some serviceis needed.

The control devices currently known, such as depicted in U.S. patentapplication, Ser. No. 08/073,238, filed on Jun. 7, 1993, in the name ofLong and assigned to the assignee of the present invention, have aseparate interlock valve for each of the solenoid valves. The use ofseparate interlock valves provides highly satisfactory operation.However, such a system not only requires more space to house the controlbut also incurs an increased cost to provide the larger housing and theadditional interlock valve.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to providean improved transmission control wherein a single interlock valveprovides the desired "lock-out" of the appropriate solenoid valve if amalfunction occurs.

It is another object of the present invention to provide an improvedtransmission control, as above, wherein conventional, but harsh,upshifting and downshifting continues after a malfunction--even afterthe driver comes to a stop and selects reverse.

It is a further object of the present invention to provide an improvedtransmission control, as above, wherein the "lock-out" is introducedwhen the transmission is shifted to reverse, thereby ensuring that thevehicle has virtually a zero forward speed to prevent any unscheduleddownshifts.

It is yet another object of the present invention to provide an improvedtransmission control, as above, wherein trim boost pressure ismaintained at an acceptable level by the differential pressure between apair of normally open pressure-regulating solenoid valves.

These and other objects of the invention, as well as the advantagesthereof over existing and prior art forms, which will be apparent inview of the following detailed specification, are accomplished by meanshereinafter described and claimed.

The present invention improves the design of existing transmissioncontrols by combining the two interlocking valves into one, therebyreducing cost and the amount of space required to house the interlockmechanism. The single solenoid interlock valve provides the desiredfunction of preventing the solenoid valves from pressurizing thegovernor and trim boost circuits while the transmission is in reverse.

A transmission shift control embodying the present invention utilizes aregulator valve means for providing a trim pressure fluid for aplurality of accumulators, including a boost plug. A shuttle valve isemployed for selectively directing pressurized fluid to first and secondends of the boost plug as well as for directing pressurized fluid to agovernor passage. A single interlock valve selectively directspressurized fluid from a first solenoid valve through the shuttle valveto the first end of the boost plug as well as through the governorpassage. The single interlock valve also selectively directs pressurizedfluid from a second solenoid valve through a shuttle valve to the secondend of the boost plug when the pressurized fluid from the first solenoidvalve is at a greater level than the pressurized fluid from the secondsolenoid valve. The boost plug is responsive to the fluid from theinterlock valve to switch the fluid from the first end of the boost plugwhen the pressure level of the fluid directed from the second solenoidvalve is greater than the pressure level of the fluid directed from thefirst solenoid valve.

To acquaint persons skilled in the art most closely related to thepresent invention, one preferred embodiment of an interlock valveembodying the concepts of the present invention and adapted for use witha transmission control--and which illustrates a best mode nowcontemplated for putting the invention into practice--is describedherein by, and with reference to, the annexed drawings that form part ofthe specification. The exemplary interlock valve is described in detailwithout attempting to show all of the various forms and modifications inwhich the invention might be embodied. As such, the embodiment shown anddescribed herein is illustrative and as will become apparent to thoseskilled in these arts, can be modified in numerous ways within thespirit and scope of the invention; the invention being measured by theappended claims and not by the details of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a diagrammatic representation of a transmission and controlembodying the present invention;

FIG. 2 is an enlarged portion of FIG. 1, the outline of which isdelineated by the chain line identified as "FIG. 2" and depicting arepresentative control valving arrangement incorporating the presentinvention in the spring-set position; and,

FIG. 3 is a further enlarged representation of the interlock valveappearing in FIG. 2, but in the pressure-set position.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT

The overall power transmission and control system embodying the conceptsof the present invention is depicted diagrammatically, and designated bythe numeral 10 in FIG. 1. The gearing portion of the transmission isrepresented by the numeral 11 and is preferably constructed inaccordance with the teaching of U.S. Pat. No. 5,009,118 issued to Ordoet al. on Apr. 23, 1991, and also assigned to the assignee of thepresent invention, although the control portion of the transmission ispreferably constructed in accordance with the teaching of the aforesaid,copending U.S. patent application, Ser. No. 08/073,238, filed on Jun. 7,1993. However, it will also become apparent that other transmissions canalso benefit from the present invention.

As depicted in FIG. 1, pressurized hydraulic fluid is provided to thecontrol system 13 by a conventional positive displacement pump 14 whichdraws hydraulic fluid from a reservoir 15 through an intake passage 17for delivery to a main line conduit 18. A conventional pressureregulator 20 controls the fluid pressure in the main line conduit 18.The excess fluid--that is, fluid not needed for transmission control andclutch operation--delivered by the pump 14 is directed by an overagepassage 21 to a conventional torque converter and clutch assembly 23. Aconventional exhaust regulator valve 22 limits the fluid pressure at thetorque converter and clutch assembly 23. The fluid flowing from theassembly 23 is directed through a cooler and lubrication system,designated as "COOLER" and "LUBE" on FIG. 1 of the drawings. Thepressure in the lubrication system circuit is established by aconventional regulator valve 26.

The main line pressure conduit 18 is connected, by branch 18_(A) with amanual selector valve 24, by branch 18_(B) to a forward-reverse controlassembly 25, by branch 18_(C) to a torque converter and clutch assembly23, and by branch 18_(D) to a first and second normally open pulse widthmodulated (PWM) solenoid valves 27 and 28 and an accumulator trim boostcontrol valve 30. The main line pressure distributed to the torqueconverter and clutch assembly 23 is utilized to engage the clutch in awell known manner. The manual selector valve 24 is adapted to bemanipulated in a well known manner to distribute the pressurizedhydraulic fluid in branch 18_(A) of the main line pressure conduit 18 inaccordance with the drive ratio selected by the operator. The selectorvalve 24 has a longitudinal bore 31 in which a spool valve member 33 isslidably disposed. The spool valve member 33 has spaced lands 34 and 35which are adapted to selectively control the flow of main linepressurized hydraulic fluid from branch 18_(A) of the main line pressureconduit 18 to a reverse passage 37 when reverse drive "R" is selected bythe operator and to a forward passage 38 when any forward drive "D1"through "D5" is selected by the operator.

When the operator desires to limit the number of forward drive ratios toless than the maximum number available (five with the depicted control),the manual selector valve 24 can be manipulated to the forward driveconditions "D4" through "D1". In the "D4" condition, main line pressureis distributed to a "D4" passage 40 as well as the forward passage 38.All of the other passages leading from the selector valve 24 areexhausted. In the "D3" condition, main line pressure is distributed to a"D3" passage 41, as well as the "D4" passage 40 and the forward passage38. In the "D2" condition, main line pressure is distributed to a "D2"passage 43 as well as the "D3" passage 41, the "D4" passage 40 and theforward passage 38. In the "D1" condition, main pressure is distributedto a "D1" passage 44 as well as the "D2" passage 43, the "D3" passage41, the "D4" passage 40 and the forward passage 38. The effect of thepressure in passages 40, 41, 43 and 44 will be hereinafter discussed inmodestly greater detail, but for an in-depth description referenceshould be made to the aforesaid U.S. patent application, Ser. No.08/073,238.

The forward passage 38 and the reverse passage 37 as well as the mainline pressure conduit 18 are distributed to the forward-reverse controlassembly 25 which is effective to establish the power flow through thetransmission in a well known manner. The aforesaid Ordo et al. patentutilizes a synchronizer to establish the forward or reverse power path.The forward-reverse control assembly 25 is preferably constructed inaccordance with the assembly described in the aforesaid U.S. patentapplication, Ser. No. 07/920,744, filed Jun. 1, 1992, in the name ofKlemen et al. and assigned to the assignee of the present invention.

The hydraulic control system 13 provides for controlling the engagementand disengagement of the friction torque transfer devices required toestablish the ratios in the transmission 11. The ratio interchangecontrol is provided by four shift valves 47A through 47D, four exhaustvalves 48A through 48D and five accumulators 502A through 50E.

As previewed in the previous paragraph, and as will appear in thedetailed description which follows, a particular structural member,component or arrangement may be employed at more than one location. Whenreferring generally to that type of structural member, component orarrangement a common numerical designation shall be employed. However,when one of the structural members, components or arrangements soidentified is to be individually identified, it shall be referenced byvirtue of a letter suffix employed in combination with the numericaldesignation employed for general identification of that structuralmember, component or arrangement. Thus, there are at least four shiftvalves which are generally identified by the numeral 47, but thespecific individual valves are, therefore, identified as 47A, 47B, 47Cand 48D in the specification and on the drawings. This same suffixconventional shall be employed throughout the specification for othercomponents.

The transmission has five friction torque transfer devices in the natureof clutches designated "C1" through "C5." One of the clutches is engagedfor each drive ratio while the remaining clutches are disengaged. Theclutch "C3" provides both the third forward speed and reverse speed. Atwo-position shuttle valve 51 is operatively connected to the properpassage of the "C3" clutch.

The spring-biased shift valve 47A controls the first/second ratiointerchange--as described in the aforesaid Long U.S. patent application,Ser. No. 08/073,238. The shift valve 47A communicates with a firstclutch feed passage 60, a first clutch apply passage 61A, a secondclutch feed passage 63A, a first clutch exhaust passage 64A, thehydraulic fluid return line 65A, the "D1" passage 44 and a governorpassage 67. The passage 60 is connected with the forward-reverse controlassembly 25 which is effective to distribute main line hydraulicpressure thereto when the forward passage 38 is pressurized.

In the spring-set position shown, the valve 47A distributes main linepressure to the clutch feed passage 60 and to the first clutch applypassage 61 to effect engagement of the clutch "C1 ". The first clutchapply passage 61A is also connected with a chamber in the accumulator50A. The accumulator 50A is effective to control the pressure rise inthe clutch "C1" during engagement in a well known manner. A trim chamberin the accumulator 50A is pressurized by a controlled pressure in a trimpassage 77, which has an effect on the pressure rise in the accumulatorchamber and therefore the engagement time of the clutch "C1" asdetermined by the pressure in first clutch apply passage 61.

In the pressure-set position (not shown) of shift valve 47A, the passage60 is connected to the second clutch feed passage 63A which is in fluidcommunication with the two/three shift valve 47B. When the spool valve47B is in the spring-set position depicted, an offset passage 78Bconnects the feed passage 63A to a second clutch apply passage 61B whichis effective, when pressurized, to enforce engagement of the secondclutch "C2". The engagement time of the second clutch "C2" is effectedby the accumulator 50B in the same manner as previously mentioned withrespect to the accumulator 50A.

The pressure in the second clutch feed passage 63A is also ported toreact with the spool valve member in the exhaust valve 48A. When theshift valve 47A is initially moved to the pressure-set position, theclutch "C1" will begin to exhaust through the restriction 88A. However,when the pressure in passage 63A is at a sufficient level, passage 64Awill connect directly to the hydraulic return line exhaust passage 90Afreely to exhaust the clutch "C1". The trigger pressure of the exhaustvalve 48A is substantially equal to the minimum pressure required forthe clutch "C2" to begin transmitting torque.

As the pressure in the governor passage 67 continues to increase, theshift valves 47C and 47D will be shifted accordingly to control thesecond/third, third/fourth and fourth/fifth ratio interchanges,respectively. The upshifting occurs in accordance with the clutchinterchange previously explained herein with respect to the first/secondratio interchange. Respective accumulators 50 and exhaust valves 48 willcontrol the timing of the clutch interchanges. It should be appreciatedthat the higher ratio clutches cannot be engaged until the next lowerclutch has first been engaged. This is commonly termed a cascadingpressure control. It should also be evident that the shuttle valve 51 iseffective to connect the passage 61C to the clutch "C3" and theaccumulator 50C during a second/third ratio interchange. Duringdownshifting when a clutch is engaged, the higher ranking clutches willbe disengaged.

If the transmission 10 is in second gear--i.e.: the valve 47A hasupshifted--and the pressure in the governor valve 67 is reduced to alevel sufficient to permit a downshift of the shift valve 47A to thespring-set position, the second clutch "C2" will be exhausted throughthe hydraulic fluid return line 65A while the clutch "C1" is engaged bypressure in passage 60. Rapid disengagement of the off-going clutchduring downshifting, is generally preferred to permit the engine tofreely accelerate to the speed required to accommodate the on-comingratio.

The reverse ratio is engaged by the manual selector valve 24 beingshifted to the reverse position to pressurize a reverse apply passage 93which is fed through the forward-reverse control 25. When the passage 93is pressurized, the shuttle valve 51 is moved to close the third clutchapply passage 61C from the shift valve 47C and simultaneously connectthe reverse apply passage 93 with the clutch "C3" and the accumulator50C. The reverse apply passage 93 is also connected with the governorinterlock valve 95 which is in fluid communication with the respectivesolenoid valves 27 and 28 for a purpose that will be hereinafterdescribed.

The solenoid valves 27 and 28 may be of the PWM type such that each iscapable of establishing a variable pressure output. The pressure outputof the solenoid valves 27 and 28 will be termed the governor pressure.However, the pressure established by each valve 27 and 28 is effected bya number of vehicle parameters including vehicle speed and throttlesetting or fuel feed. Other parameters may be provided as desired. Thesignals controlling the PWM solenoid valves 27 and 28 are preferablyestablished by a conventional pre-programmed digital computer which isincorporated in the transmission and control 10 and programmed in aconventional manner to establish the various pressures. Such computersand the operating or control algorithms are well known. The solenoidvalve 27 is controlled at one pressure schedule while the solenoid valve28 is controlled at another pressure schedule during upshifting. Asshould be appreciated, the pressure output of the solenoid valve 27increases before the pressure output of the solenoid valve 28. Thepurpose for these different schedules will be explained in conjunctionwith the description of the valves 30 and 95.

As best seen in FIG. 2, the governor interlock valve 95 includes a spoolvalve member 100 having axially spaced lands 101,102, 103 and 104slidably disposed in a bore 105. Lands 101 and 102 cooperate with thebore 105 to define an interlock chamber 106. The interlock chamber 106communicates with the hydraulic fluid return line 135A when the spoolvalve member 100 is in the pressure-set position to exhaust anyhydraulic fluid that might seep into the interlock chamber 106 pasteither land 101 or land 102.

The spool valve member 100 is urged toward one end of the bore 105 by aspring 108 disposed in chamber 109 and compressed between a plug 110 andthe land 104. In the spring-set position represented in FIG. 2, theoutput pressure of solenoid valve 27 is in fluid communication, throughpassage 111, with the bore 105 between the lands 102 and 103 to providecontrolled fluid pressure to a control passage 113 which is in fluidcommunication with a control chamber 114 provided in a governor shuttlevalve 115. A governor feed passage 117 branches from the control passage113 and is also connected with the shuttle valve 115 between lands 14 1and 143 when the spool valve member 136 is disposed in the positiondepicted in FIG. 2. Similarly, the output pressure of the solenoid valve28 is in fluid communication, through passage 112, with the bore 105between the lands 103 and 104 to provide control fluid pressure to thecontrol passage 131 which is in fluid communication with a controlchamber 133 formed on the shuttle valve 115. A governor feed passage 134branches from the passage 131 and is also connected with the shuttlevalve 115.

The shuttle valve 115, as best seen in FIG. 2, includes a spool valvemember 138 having three lands 140, 141 and 143 which are slidablydisposed in a bore 144 between the chambers 114 and 133. The bore 144 isdisposed in fluid communication with the passages 117 and 134 as well asa pair of governor pressure passages 145 and 147, and a secondary trimboost passage 148. Both governor pressure passages 145 and 147 areconnected with a primary boost passage 150 which, in turn, communicateswith the primary governor passage 67.

The spool valve member 138 in shuttle valve 115 is positioned by thepressure in the opposed chambers 114 and 133. It should be explainedthat the pressure from solenoid valve 27 increases before the pressurefrom solenoid 28. Therefore, the shuttle valve 115, during normalforward operation, will be disposed in the position shown in FIG. 2. Sodisposed, the pressure from solenoid 27 is directed via passages 113,117, 145 and 150 to the governor passage 67. The passage 147 is closedat the land 140 and the output pressure of the solenoid valve 28 isdirected via passages 131 and 134 to the secondary boost passage 148.

The boost passage 148 is connected for fluid communication with one sideof a boost plug 151, and boost passage 150, through governor passage 67is connected for fluid communication with the opposite side of boostplug 151. The boost plug 151 is a component of the accumulator trimboost valve 30, as depicted in FIG. 2. The plug 151 cooperates with abore 153 to define a primary chamber 154, connected with governorpassage 67 and thereby indirectly with primary boost passage 150. Asecondary chamber 155 is connected in fluid communication with thesecondary trim boost passage 148. The accumulator trim boost valve 30also includes a regulator valve portion 157 which is connected with theplug 151 through a pin and stop 158, and a spring 160. The regulatorvalve portion 157 includes a valve spool member 161 having spaced lands163 and 164 slidably disposed in a bore 165. The bore 165 is connectedwith the main line pressure conduit branch 18_(D), the trim passage 77and a pair of exhaust or hydraulic fluid return lines 167. The trimpassage 77 is connected to a control chamber 170 defined between thelands 163 and 164 through a restriction 168.

Fluid pressure in trim passage 77 will urge the spool member 161 againstthe spring 160 in a direction to close the main line pressure conduitbranch 18D at land 163 and open the exhaust passage 167 previouslyclosed by land 164. This action will control the pressure in the trimpassage 77 in a well known manner. Fluid pressure operating on the plug151 will control the amount of compression in the spring 160 andtherefore, the pressure level at which the regulator valve 157 maintainsthe pressure in trim passage 77. When the plug 151 is urged against thespring 160 by pressure in primary chamber 154, as determined by thesolenoid valve 27, the pressure level in trim passage 77 will be at ahigh level, and when both chambers 154 and 155 are pressurized, thepressure level in trim passage 77 will be at a low level. The pressurelevel in trim passage 77 will provide a bias pressure for the trimchamber 74 in the accumulators 50 (through trim passage 77)--therebyproviding a control pressure for the clutches "C1" through "C5" in awell known manner.

The interlock valve 95 is connected with the reverse passage 93 which,as previously explained, is pressurized through the manual selectingvalve 24 and the forward-reverse control 25 when the reverse drive isselected by the operator. Specifically, the reverse passage 93communicates with the face of the land 101 to impose a bias pressurethereon whenever the reverse drive is selected.

When the reverse passage 93 is pressurized, the spool member 100 will bemoved to its pressure-set position represented in FIG. 3 by the fluidpressure acting against land 101. In the pressure-set position, theexhaust passage 135A will be closed by the land 101, and the passage 111from solenoid valve 27 will be closed by land 102. Also in thepressure-set position, passage 113 from shuttle valve 115 will beconnected to exhaust passage 135B between lands 102 and 103. Similarly,in the pressure-set position of spool member 100, the passage 131 fromthe shuttle valve 115 will be connected with the exhaust passage 135C.

OPERATION

With the vehicle engine operating, the pump 14, in conjunction with theregulator 20, provides pressurized hydraulic fluid. With the selectorvalve position for "D5", the vehicle will respond to a throttle increaseby the operator to provide forward motion.

The transmission can be limited to less than all of the forward speedratios by manipulation of the manual valve. For example, if the operatordoes not wish the transmission to reach the fifth forward speed ratio,the manual valve will be moved to the "D4" position. In this position,the "D4" passage 40 will be pressurized. The pressure in this passage 40is directed to the valve 47D. This pressure acts to assist in resistingthe upshifting of the valve 47D. The pressure in the passage 67 will notbe sufficient to force the upshifting of the valve 47D.

Manipulation of the manual valve to the other forward ratio positionsrepresented by "D3" through "D1" will result in limiting the upshiftingof the transmission to the third forward through first forward ratios,respectively. In "D3", the passage 41 will be pressurized to prevent theupshifting of the valve 47C such that the transmission control cannotenergize the clutches "C4" and "C5". Likewise, the pressurization of therespective passages 43 and 41 prevents the shifting of valves 47B and47A, respectively. The operator can control the upshifting to someextent by starting in "D1" and upshifting to successive gears asdesired. The upshift will occur if the other parameters aresatisfied--that is, if the pressure in passage 67 is sufficient to shiftthe respective valves. The operator can downshift from any forward to alower forward ratio through the manipulation of the manual valve 24.

The purpose of the shuttle valve 115 is to direct the higher pressureoutput of the two solenoid valves 27 and 28 to the lower end of the plug151 and to the governor passage 67. When the control system is operatingas intended, the solenoid valve 27 will, at a predetermined portion ofthe cycle, provide a higher pressure than the solenoid 28 during theupshift cycle. However, if the solenoid valve 28 should inadvertentlyproduce a higher pressure than the solenoid valve 27, the chamber 133will be at a higher pressure resulting in the shuttle valve 115 beingforced into the chamber 114. In this position, the passage 131 isconnected to the passage 147 between lands 140 and 141 and the passage117 is connected with the passage 148 between the lands 141 and 143.Thus, it should be evident that the output pressures of the solenoidvalves 27 and 28 and their function is then reversed.

If either solenoid valve 27 or 28 should malfunction and provide aconstant output pressure other than zero, the normal shift sequenceswill be interrupted. The operator can eliminate this situation bybringing the vehicle to a stop and shifting to reverse. When the reverseratio is selected, the reverse apply passage 93 is pressurized. Thisresults in the shuttle valve 91 being moved to direct the fluid pressurein passage 93 to the clutch "C3". Also during a shift to reverse, theforward-reverse control 25 will condition the necessary mechanism (i.e.:a synchronizer) to the proper position.

The pressure in passage 93 will also act on the land 101 to shift thespool valve member 100 against the spring 108 to its pressure-setposition (FIG. 3). When the spool valve member 100 is in thepressure-set position, the output pressures from the solenoid valves 27and 28 are blocked by lands 101 and 102, respectively. If one of thesolenoid valves has malfunctioned in a high output pressure condition,the interlock valve 95 will remain in the shifted position because ofthe pressure bias from the reverse passage 93 against land 101. When theoperator shifts to a forward drive condition, and while the spool valvemember 101 will then return to the spring-set position, only onesolenoid valve pressure output will be available to the governor passage67 and boost passage 150. This will result in maximum trim boostpressure at the accumulators 50 such that the operator will experienceharsh shifting at all throttle controls. This shift feel willcontinually remind the operator that some repair is required. However,the operator will have the entire range of operation available until therepairs are effected.

The foregoing description of the exemplary embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhausted or to limit the invention to the preciseform disclosed. Obvious modifications or variations are possible inlight of the above teachings. The embodiment was chosen and described toprovide the best illustration of the principles of the invention and itspractical application to thereby enable one of ordinary skill in the artto utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. All suchmodifications and variations are within the scope of the invention asdetermined by the appended claims when interpreted in accordance withthe breadth to which they are fairly, legally and equitably entitled.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A transmission shiftcontrol comprising:regulator valve means for providing a trim pressurefluid for a plurality of accumulators through a boost control valvehaving a plug means; said plug means having first and second ends;shuttle valve means for selectively directing pressurized fluid to saidfirst and second ends of said plug means and for directing pressurizedfluid to a governor passage; first and second solenoid valve means; asingle interlock valve for selectively directing pressurized fluid fromsaid first solenoid valve means through said shuttle valve means to saidfirst end of said plug means and said governor passage and forselectively directing pressurized fluid from said second solenoid valvemeans through said shuttle valve means to said second end of said plugmeans when the pressurized fluid from said first solenoid valve means isat a greater pressure level than the pressurized fluid from said secondsolenoid valve means; and, said plug means being responsive to the fluidfrom said interlock valve to switch the fluid from said first end ofsaid plug means when the pressure level of the fluid directed from saidsecond solenoid valve means is greater than the fluid directed from saidfirst solenoid valve means.
 2. A transmission shift controlcomprising:manual valve means for directing fluid to establish forwardand reverse drive ratios; regulator valve means for providing a trimpressure fluid for a plurality of accumulators through a boost plugmeans having a plug means; said plug means having first and second ends;a governor passage; shuttle valve means for selectively directingpressurized fluid to said first and said second ends of said plug meansand for directing pressurized fluid to said governor passage; first andsecond solenoid valve means; a single interlock valve for selectivelydirecting pressurized fluid from said first solenoid valve means throughsaid shuttle valve means to said first end of said plug means as well asto said governor passage; said interlock valve also selectivelydirecting pressurized fluid from said second solenoid valve meansthrough said shuttle valve means to said second end of said plug meanswhen the pressurized fluid from said first solenoid valve means is at agreater pressure level than the pressurized fluid from said secondsolenoid valve means; said plug means being responsive to the fluid fromsaid interlock valve to switch the fluid from said interlock valve tosaid first end of said plug means when the pressure level of the fluiddirected from said second solenoid valve is greater than the fluiddirected from said first solenoid valve; and, said interlock valveblocking fluid flow from both said first and said second solenoid valvemeans when the reverse drive ratio is selected by said manual valvemeans.